A method of anisotropic etching of silicon by cyclic etch and deposition steps in a plasma environment is known as “Bosch process” from German patent DE4241045, issued 5 Dec. 1992 and U.S. Pat. No. 5,501,893, issued 26 Mar. 1996, titled “Method of anisotropically etching silicon”. This anisotropic plasma etching has been applied to other materials such as Ge, SiGe and GasAs.
When etching high-aspect-ratio silicon features using the “Bosch process” (DRIE--deep reactive ion etching), researchers found that there is a maximum achievable aspect ratio, “critical aspect ratio”, of an etched silicon trench. At this critical aspect ratio, the apparent etch rate defined as the total depth etched divided by the total elapsed time no longer monotonically decreases as the aspect ratio increases, but abruptly drops to zero.
Junghoon Yeom, et al, in the paper titled “Maximum achievable aspect ratio in deep reactive ion etching of silicon due to aspect ratio dependent transport and the microloading effect”, J. Vac. Sci. Technol. B, Vol. 23, No. 6, November/December 2005, 2319-2329, proposed a theoretical model to predict the critical aspect ratio and reveal its causal mechanism. The model considers aspect ratio dependent transport mechanisms specific to each of the reactant species in the three subprocesses of a time-multiplexed etch cycle: deposition of a fluorocarbon passivation layer, etching of the fluorocarbon polymer at the bottom of the trench, and the subsequent etching of the underlying silicon. The model predicts that the critical aspect ratio is defined by the aspect ratio at which the polymer etch rate equals the product of the deposition rate and the set time ratio between the deposition and etching phases for the time-multiplexed process. Several DRIE experiments were performed to qualitatively validate the model. Both model simulations and experimental results demonstrate that the magnitude of the critical aspect ratio primarily depends on (i) the relative flux of neutral species at the trench opening, i.e., the microloading effect, and (ii) aspect ratio dependent transport of ions during the polymer etching subprocess of a DRIE cycle.
This result means that thick sidewall passivation layers limit “critical aspect ratio” achievable with DRIE process. This effect becomes especially important for nanosize structures where high aspect ratio feature etch requires extremely thin passivation layers.
C. Craigie et al, in the paper titled, “Polymer thickness effects on Bosch etch profiles”, J. Vac. Sci. Technol. B 20.6., November-December 2002, 2229-2232, reported thick polymer films developing on Bosch etched features as a result of both long deposition times and high C4F8 flow rates. Polymer builds up on the sidewalls of trenches. Where polymer thickness is a significant proportion of the trench width, the incident ion flux is restricted. Narrow trenches converge more rapidly than wide ones, as a greater proportion of the width is obscured.
The regular thickness of sidewall polymer passivation layers in state-of-the-art DRIE process is larger than 1000 Å, as reported by J. Reimers et al in the Danish technical Institute Report titled “Fabrication of a Microfluidic System for Magnetic Separation using Deep Reactive Ion Etching”, 2004. In some cases such sidewall polymers are reported in micrometer ranges, for example in abovementioned paper of C. Craigie. Alcatel Vacuum Technology at the website, www.adizen.com (M. Puech, et al, “A Novel Plasma Release Process and Super High Aspect Ratio Process using ICP Etching got MEMS”) proposed so called “SHARP” process (Super High Aspect Ratio Process), consisting of inserting a specific passivation removal step by oxygen plasma. This passivation removal process is 5 times faster than traditional SF6-based process step. It was demonstrated that this special passivation layer removal step, performed prior to etching step, allowed to achieve aspect ratio etch as high as 60.
Martin Walker from Oxford Technology in the paper titled “Comparison of Bosch and cryogenic processes for patterning high aspect ratio features in silicon”, published in Proceedings of SPIE, Volume 4407, MEMS Design, Fabrication, Characterization, and Packaging, April 2001, pp. 89-99, describes another problem with Bosch passivation process, when it is applied to high aspect ratio feature etch: As the ratio of depth to width increases, so the attack on the sidewalls due to ion bombardment becomes less. This can result in build-up of polymer, resulting in negative profiles if it occurs at the top of the hole, or the formation of grass when it occurs at the bottom etched surface.
The same author describes so called “cryo” process as an alternative to Bosch process. Just as for the Bosch process, this technique also uses SF6 to provide fluorine radicals for silicon etching. The silicon is removed in the form of SiF4, which is volatile. The main difference is in the mechanism of sidewall passivation and mask protection. Rather than using a fluorocarbon polymer, this process relies on forming a blocking layer of oxide/fluoride (SiOxEy) on the sidewalls (around 10-20 nm thick), together with cryogenic temperatures inhibiting attack on this layer by the fluorine radicals. The low temperature operation also assists in reducing the etch rate of the mask material, which is normally either photoresist or silicon dioxide. The attack on these materials by free radical fluorine is chemical in nature and is sensitive to temperature, with the etch rate dropping rapidly at cryogenic temperatures. The low temperature can have a bad effect on some organic materials, causing cracking. This is more severe for thicker photoresists than for thin layers. As a rough guideline, layers of resist used for this process should not be more than 1.5 μm thick, to avoid the hazard of cracking.
Yi Zhao et al, in the paper titled “Creating Silicon nanostructures with Controllable Sidewall Profiles by Using Fluorine-Enhanced Oxide Passivation”, The 10th International Conference on Miniaturized Systems for Chemistry and Life Sciences (uTAS2006), Nov. 5-9, 2006, Tokyo, Japan, suggested an alternative to the “cryo” process by alternating reactive ion etching and air exposure: During plasma etching, F being adsorbed on the surface breaks preexisting Si—O bonds and form Si—F group, thus forming a SiOxFylayer. When exposed to the coexistence of oxygen and moisture, the Si—F groups are quickly replaced by Si—OH groups from the water vapor ambient due to the similar ionicity. The Si—OH groups bridge and form layers of oxide. The oxide continues to grow due to field assisted Mott-Cabrera mechanism. The fluorine-enhanced oxide layer postpones the attack of the sidewalls during the following brief plasma etching. The sidewall passivation is thus enabled. Obviously, control of sidewall passivation layer thickness and uniformity is very problematic with this method.
Suk Won Yu has used dielectric etch preventing spacer deposited on the sidewall of the etched feature in KR 10-2006-0030717. Unfortunately, due to the chosen deposition method such spacer is quite thick and non-uniform along the length of the sidewall feature, as it clearly seen from the patent's text and the drawing. Mr. Yu does not teach how to get uniformly deposited sidewall passivation layer, which will be thin enough and uniform enough for nano-sized features with high aspect ratio.